The present invention relates to a semiconductor laser device having a greatly stabilized oscillation wavelength to output a high output laser beam and a semiconductor laser module.
A demand for increasing the capacity of optical communication networks has been recently heightened because of a proliferation of various multimedia including Internet-delivered multimedia. Responsive to the demand, a WDM (Wavelength Division Multiplexing) communication system has been used. The WDM communication system is a system for performing transmission by using a plurality of wavelengths in a 1,550-nm band. The WDM communication system realizes a remarkable increase of the transmission capacity of a network because optical signals having a plurality of different wavelengths are simultaneously transmitted through one optical fiber and thereby it is unnecessary to construct a new line.
In the case of the light source, or the amplifying light source, of the optical signal, it is desired to accurately control the oscillation wavelength and operate a semiconductor laser element at a high optical output by preventing the thermal saturation of the element. Conventional semiconductor laser devices prevent an oscillation wavelength from becoming unstable, and a semiconductor laser element for outputting a laser beam from being thermally saturated, by setting a thermistor for measuring the temperature of the semiconductor laser element nearby the semiconductor laser element and controlling the temperature of the semiconductor laser element by a temperature controlling element such as a Peltier element.
FIG. 15 is a perspective view of a schematic configuration of a conventional semiconductor laser device. In the case of the semiconductor laser device in FIG. 15, a submount 102 formed by AlN having an insulating characteristic and a high heat conductivity is formed on a carrier 101 formed by CuW and a semiconductor laser element 103 for outputting a laser beam L100 having a predetermined wavelength is formed on the submount 102. A submount 104 formed by AlN is formed on the carrier 101 and a thermistor 105 for measuring the temperature of a semiconductor laser element is formed on the submount 104.
The semiconductor laser element 103 and the submount 102 are joined through a metallic thin film 102a. The metallic thin film 102a is metallized with Ti, Pt, and Au at film thicknesses of 60, 200, and 600 nm in order and the semiconductor laser element 103 and submount 102 are joined to each other by a solder material made of AuSn or the like metallized on the Au film. Moreover, the thermistor 105 and submount 104 are joined each other similarly through the metallic thin film 104a. 
In the case of the semiconductor laser element 103, the face to be joined with the submount 102 serves as a p-side electrode and the upper face serves as an n-side electrode and an active layer for mainly generating heat is set nearby the submount 102. A negative electrode is led to the n-side electrode by an Au wire 106a and the p-side electrode is led to the positive side carrier 101 through the metallic thin film 102a and an Au wire 106b. 
Referring to FIG. 16, the submount 102 secures the insulation of the semiconductor laser element 103 and functions as a heat sink of the semiconductor laser element 103 and is joined to a CuW base 106 to be joined to the bottom of the carrier 101 by AuSn solder, and a Peltier module 107 set to the bottom of the base controls the temperature of the semiconductor laser element 103 in accordance with the temperature detected by the thermistor 105.
Moreover, the thermistor 105 is also insulated from the carrier 101 by the submount 104 similarly to the case of the semiconductor laser element 103 to detect the temperature of the semiconductor laser element 103 through the submount 102, carrier 101, and submount 104 respectively having a high heat conductivity.
When performing long distance optical transmission by using the above WDM communication system, it is desireable to increase the output of a laser beam of a signal light source in order to increase the interval between repeaters. Moreover, to improve the amplification capacity of an optical fiber amplifier, it is desirable to increase the output of a semiconductor laser device used for an exciting light source.
To meet the above demands, a conventional embodiment has a semiconductor laser element for oscillating and outputting a laser beam of 250 mW or more as a laser beam for Erbium doped fiber amplifier (EDFA) excitation. However, a conventional semiconductor laser device using the above high output semiconductor laser element has a problem in that the optical output and service life of the semiconductor laser element are deteriorated.
FIG. 16 shows a front view of the above conventional semiconductor laser device including the above described base and Peltier module. The submounts 102 and 104 are separately provided for the semiconductor laser element 103 and the thermistor 105. The heat generated in the semiconductor laser element 103 is conducted to the thermistor 105 through the metallic thin film 102a, submount 102, metallic thin film 102b, carrier 101, metallic thin film 104b, submount 104, and metallic thin film 104a in order as shown by the arrow YA in FIG. 16. As recognized by the present inventors, because the heat conducting distance is physically increased as described above, detection of the actual temperature of the semiconductor laser element 103 is delayed.
Moreover, because the total of four junction faces corresponding to the metallic thin films 102a, 102b, 104b, and 104a are present on the heat conducting path between the semiconductor laser element 103 and the thermistor 105, heat resistances are generated on these junction faces and thereby, the temperature of the semiconductor laser element 103 is not accurately transferred to the thermistor 105. That is, the thermistor 105 detects a lower temperature having a large difference from the actual temperature of the semiconductor laser element 103 and therefore, the accuracy of a detected temperature is deteriorated. As a result, the temperature control of the semiconductor laser element 103 performed in accordance with the temperature detected by the thermistor 105 is delayed and because the temperature control at a low accuracy is inevitably performed, the oscillation wavelength of the semiconductor laser element 103 becomes unstable and the optical output and service life of the semiconductor laser element 103 are deteriorated.
Moreover, as shown by the arrow YB in FIG. 16, because the total of four junction faces such as two junction faces corresponding to the metallic thin films 102a and 102b, the junction face between the carrier 101 and the base 106, and the junction face between the base 106 and the Peltier module 107 are present on the heat conducting path between the semiconductor laser element 103 and the Peltier module 107, the heating action or cooling action by the Peltier module 107 is deteriorated whenever passing through these junction faces and resultantly, the temperature control of the semiconductor laser element 103 cannot be quickly or accurately performed.
Moreover, when supplying a current of 1 A or more to a semiconductor laser element in order to obtain a high optical output and assuming that the total resistance of an Au thin film in the metallic thin film 102a is 0.12xcexa9, the voltage drop by the Au thin film becomes 0.12 V. Moreover, because the inter-electrode voltage of a semiconductor laser element is approximately 2 V when a current of 1 A is supplied to the semiconductor laser element, the voltage drop of the semiconductor laser element in the resonator length direction becomes un-uniform by 0.12 V. That is, also when considering a current injection path, current injection into a semiconductor laser element becomes ununiform because of passing through a metallic thin film and resultantly, the light density in the active layer becomes ununiform and it is estimated to accelerate the deterioration of the optical output and service life of the semiconductor laser element.
When using a high output semiconductor laser element, it is recognized by the present inventors that it is imprudent to ignore the voltage drop in the metallic thin film between the semiconductor laser element and a submount. If too high, an oscillation wavelength becomes unstable due to the non-uniformity of the voltage drop in the metallic thin film in the resonator length direction. This problem particularly becomes troublesome in the case of a semiconductor laser element designed for a high output operation at a resonator length of 1,000 xcexcm or more.
It is an object of the present invention to address the above-described and other deficiencies of conventional devices and modules.
It is another object of this invention to provide a semiconductor laser device and a semiconductor laser module capable of preventing the optical output and service life of a high output semiconductor laser element from deteriorating. This is achieved by using new materials to reduce the number of interfaces between a laser element and a temperature measuring device, as well as a reduction in the number of interfaces between a laser element and a Peltier module. In addition, performance is enhanced by improving the uniformity of voltage drop of the semiconductor laser element in the resonator length direction by distributing driving currents along a length of the resonator cavity.
The first aspect of the present invention includes a semiconductor laser device including a semiconductor laser element for outputting a laser beam, a temperature measuring element for measuring the temperature of the semiconductor laser element, and a carrier having an insulating characteristic and a high heat conductivity, wherein the semiconductor laser element and the temperature measuring element are joined to the carrier through a multi-layer film that includes a gold thin film and are arranged closely to each other such that the heat resistance of a heat conducting path decreases. Within the first aspect, variations in the pattern of the multi-layer film are possible. In addition, the carrier may also include rods of thermally conductive material.
The second aspect of the present invention includes a semiconductor laser device including a semiconductor laser element for outputting a laser beam, a temperature measuring element for measuring the temperature of the semiconductor laser element, a carrier having an insulating characteristic and a high heat conductivity, and a common laser and temperature measure device submount having a heat conductivity of 500 W/(mxc2x7K) or more joined to the carrier through a multi-layer film that includes a gold thin film. The semiconductor laser element and the temperature measuring element are each joined to the common submount via multi-layer film that includes a gold thin film and are arranged closely to each other such that the heat resistance of a heat conducting path decreases.
The third aspect of the present invention includes a semiconductor laser device including a semiconductor laser element for outputting a laser beam, a temperature measuring element for measuring the temperature of the semiconductor laser element, an optical monitor for monitoring the output of the laser element, and a carrier having an insulating characteristic and a high heat conductivity, wherein the semiconductor laser element, the temperature measuring element and the optical monitor are joined to the carrier through a multi-layer film that includes a gold thin film and are arranged closely to one another such that the heat resistance of a heat conducting path decreases.
The fourth aspect includes a semiconductor laser device having a semiconductor laser element for outputting a laser beam, a temperature measuring element for measuring the temperature of the semiconductor laser element, a carrier having an insulating characteristic and a high heat conductivity, and laser submount having a heat conductivity of 500 W/(mxc2x7K) or more and a joined to the carrier through a multi-layer film that includes a gold thin film. The temperature measuring element is joined directly to the carrier through a multi-layer film that includes a gold thin film and is arranged closely to the semiconductor laser device such that the heat resistance of a heat conducting path decreases. Within the fourth aspect, variations regarding the size and other characteristics of the laser submount are possible.
According to the first through fourth aspects, the heat resistance of the heat conducting path is decreased by using three or less junction faces located on the heat conducting path between the semiconductor laser element and the temperature measuring element and respectively formed by the multi-layer film including the gold thin film.
The fifth aspect of the present invention includes a semiconductor laser module having the semiconductor laser device of any of the above aspects and a temperature controlling element for controlling the temperature of the semiconductor laser element in accordance with the temperature output from the temperature measuring element. In this aspect, the carrier of the semiconductor laser device is joined to the temperature controlling element and the semiconductor laser element is temperature controlled through the carrier.